Instagram carousels were stored as content_type=instagram_carousel, which the
publisher's match() did not handle — publishing failed with "Unsupported
Instagram content type: instagram_carousel" for any post created via API, MCP,
or template (the AI flow worked only thanks to an inline band-aid that rewrote
carousel to feed before saving).
A carousel is just an Instagram feed post with multiple images: the editor,
preview, and publisher already treat a multi-image feed as a carousel. So
instagram_carousel is a generation format, not a stored content type. Remove it
from the ContentType enum entirely; it now lives only as an AI generation-format
string (wizard card + slide structure + carousel templates are untouched), and
posts always persist as instagram_feed.
- ContentType: drop the InstagramCarousel case; InstagramFeed maxMediaCount 1 -> 10
- StreamPostCreation: resolvedContentType() maps carousel -> feed; band-aid removed
- StartPostCreationRequest: accept instagram_carousel as a generation format
- Frontend: carousel becomes a wizard-local AiFormat; UI/UX unchanged
- API/MCP now reject instagram_carousel as a content_type (Rule::in no longer lists it)
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